Various types of processes have been utilized for beneficiating ores. The type of process utilized generally depends, not only on the nature of the ore, but also on the range of sizes of raw input to be processed. For instance, in beneficiating coal ore, the larger sizes of raw input are separated in jigs, dense medium baths, cyclones, and hydrocyclones. These processes have been used effectively to beneficiate coal ore having a size range in excess of 28 mesh. For smaller coal ore sizes, such as 28 mesh.times.0, froth flotation processes have usually been used to effect the desired separation of coal from refuse. Generally, a typical coal cleaning plant employs several of these processes in tandem to beneficiate a full range of raw input sizes.
As well known in the art, froth flotation processes utilize differences in the surface characteristics of the ore and the refuse to effect separation. When used to separate coal from refuse, the process takes place in a large vat or cell containing a separating medium which is composed largely of water with a small amount of fuel oil and froth promoter. The finely divided raw input is charged into the cell and circulated therein as compressed air is injected at spaced locations into the bottom of the cell. Because the coal particles are hydrophobic, and the refuse particles are hydrophilic, the air bubbles tend to associate with the coal particles, causing them to float to the surface of the cell and form a froth which is continuously skimmed and removed for further processing. The hydrophilic refuse particles settle to the bottom of the cell from which they are removed and conveyed to refuse handling equipment.
The conventional froth flotation process takes place in a series of open cells of whatever number maybe required to process a given amount of raw input per unit of time. Conventional froth flotation cells, however, have a number of disadvantages. For instance, most rely on the random motion of the bubbles through the separating medium to find and attach themselves to the coal particles in the raw input. As a result, some air escapes without ever contacting coal particles. Because compressed air is an expensive commodity, it should be apparent that a froth flotation process which utilizes compressed air more efficiently and effectively is highly desirable. Heretofore, several froth flotation stages have been required to overcome this inefficiency and to produce satisfactory yields.
Conventional froth flotation cells are open to the atmosphere. Because the frothing agents used are volatile, they escape into the ambient air around the cells and thus pollute the environment. Such volatility also limits the types of frothing agents and separating mediums which can be used in froth flotation processes.
Various techniques have been proposed in the art to improve the efficiency of the froth flotation process. For instance, one known process for improving aeration utilizes a nozzle to spray a raw input slurry or pulp at a high velocity onto the surface of the separating medium in a froth flotation cell. The flight of the pulp stream through the air and the turbulence created at its point of impact aerate the raw input. See U.S. Pat. Nos. 2,416,066 and 2,850,164. Mechanical mixing devices utilizing impellers, and hydraulic mixing utilizing baffling techniques have been proposed, as may be seen in U.S. Pat. No. 3,015,396. Other proposed systems, as disclosed in U.S. Pat. No. 3,400,818, include the use of vortex flow patterns along with air intake suction tubes to effect the desired aeration. U.S. Pat. No. 2,746,605 discloses a froth flotation process wherein air is injected into a slurry in a mixing nozzle. U.S. Pat. No. 2,641,362 discloses a slurry aeration nozzle wherein air is inducted into the flowing slurry. U.S. Pat. Nos. 4,253,942 and 1,367,223 are also of interest regarding devices for aerating slurries.
Other efforts have been made to improve the efficiency and effectiveness of hydraulic ore separation processes, including froth flotation processes.
For example, U.S. Pat. No. 2,641,362 discloses froth flotation apparatus wherein air is inducted into a recirculating flow of medium and conditioned slurry is admitted into the top of a separating vessel.
U.S. Pat. No. 3,428,175 discloses a froth flotation column having an enclosed upper end from which low density particles are removed by means of a vacuum source connected through a cyclone. An aerated and conditioned slurry is admitted tangentially into the column below the frothing section, and additional flotation air is admitted into the bottom of the column for movement upwardly and outwardly through a conical transition zone.
U.S. Pat. No. 3,071,447 discloses yet another type of hydraulic classifier in which closed apparatus is used to practice a crystallization process.
U.S. Pat. No. 3,730,341 discloses a froth flotation column wherein pretreated slurries are admitted into the bottom of a conical section and flowed upwardly in a shaped annular space formed between the inside wall of the column and a centrally located flow restrictor.
U.S. Pat. No. 4,253,942 discloses another type of flotation separator utilizing a flotation column wherein the flow of a pretreated slurry admitted adjacent to the bottom is regulated by means of the column configuration and a specially designed flow restrictor mounted therein.
U.S. Pat. No. 2,746,605 discloses a froth flotation device utilizing air injected into a pretreated slurry flowing into an open tank.
U.S. Pat. No. 4,287,054 discloses another type of froth flotation device wherein air is aspirated into flowing water which is injected into the base of a cylindrical column wherein feedstock is charged at an upper level.
U.S. Pat. No. 1,869,241 discloses a froth flotation column utilizing various types of flow restrictors which cooperate with an aerated flotation emulsion admitted at about the midpoint of the column to interact with ore charged downwardly into the column.
U.S. Pat. No. 1,176,428 discloses a froth flotation tank having a closed head space from which particle-carrying froth is exhausted by means of a vacuum. Air bubbles are admitted into the tank through a porous bottom, and the slurry is charged into the tank at a different location.
U.S. Pat. No. 3,446,353 discloses another type of froth flotation apparatus wherein a slurry mixture is flowed horizontally into the lower end of a tank and impinges upon an upstanding baffle near the bottom of the tank.
U.S. Pat. No. 1,187,772 discloses a froth flotation process and apparatus wherein feedstock is admitted downwardly into the upper end of a tank simultaneously with the injection of a frothing agent and a carrying medium.
U.S. Pat. Nos. 864,856 and 873,586 both disclose vertical column devices for beneficiating ores.
Other separating devices of interest are disclosed in U.S. Pat. Nos.: 3,506,120; 2,922,521; 3,202,281; and 1,180,089.
Coal cleaning processes of interest are disclosed in U.S. Pat. Nos.: 4,244,699; 4,274,946; and 4,270,926.
While the above-noted patented processes and apparatus may function satisfactorily for their intended purposes, none is entirely satisfactory from both an efficiency and environmental standpoint.